The subject matter of the invention is a process for the production of a fluorophosphate glass free of beryllium with a refractive index n.sub.e between 1.40 and 1.46, an Abbe value v.sub.e between 85 and 94 and a high anomalous partial dispersion value +.increment.v.sub.e between 19 and 28.
In correcting the so-called "secondary spectrum" of special optical devices the computing optician heretofore had been restricted to the supplemental use of natural or synthetic crystals, for example, the mineral fluorite (CaF.sub.2). Because the properties of the material are inadequate for advanced manufacturing methods, for example, complete cleavability on the octahedral planes and the relatively low hardness, attempts have been made recently to develop optical glasses with optical and physical properties similar to those of cubic fluorite, but with better chemical and processing properties. This involves not only the parameter constellation of the refractive index n.sub.e (fluorite: n.sub.e =1.4349) and the Abbe value v.sub.e (fluorite: v.sub.e =94.8), but also additionally the anomalous partial dispersion value expressed as the deviation (positive or negative) from the so-called "standard line" .increment.v.sub.e. This value in the case of fluorite amounts to .increment.v.sub.e =+31.5.
Fluorophosphate based glasses having additionally positive anomalous partial dispersion values are already known. They are, however, outside the range of the present invention with respect to their n.sub.e -v.sub.e parameter pairs. Thus, a glass with the values of n.sub.D =1.38 and v=95.5 is known from CA-P 570 108, a fluoride glass with n.sub.D =1.35 and v=81 from U.S. Pat. No. 2,716,069, a glass with n.sub.d =1.33 and v.sub.e =105 from GB-P 845 952 and a fluoride glass with n.sub.D =1.39 and v.sub.e =99.5 from U.S. Pat. No. 2,511, 224.
Aside from the fact that the optical positional ranges of these glasses in the n-v diagram differ from those of the glasses produced by the process of the invention, the known glasses without exception contain beryllium fluoride (BeF.sub.2) as the glass former. Due to the toxicity of the beryllium compounds, the use of these components of the mixture may be harmful to health during the melting process and the subsequent processing of the glass.
Even though a fluorine containing phosphate glass free of alkalies and beryllium is known from DE-P 1 088 674--it may also contain small additions of B.sub.2 O.sub.3 --the optical properties of the glass with refractive indices n.sub.d between 1.536 and 1.589 and Abbe values v between 66.4 and 72.8 are in an entirely different area of the optical diagram. This is also true for the fluorophosphate glasses known from DE-AS 2 342 484 with n.sub.e &gt;1.57 and v.sub.e &lt;70; for the glasses described in DE-P 1 596 877 with n.sub.e between 1.47 and 1.53 and v.sub.e between 75 and 85 and for an optical glass known from DE-P 1 496 566 with n.sub.e values between 1.53 and 1.55 and v.sub.e values between 62 and 74.
Finally, fluorophosphate glasses have already been described, but these contain as a further component, among others, B.sub.2 O.sub.3, for these see U.S. Pat. No. 3,656,976 (n.sub.e :1.47 to 1.54; v.sub.d : 84.2 to 71.6) and DE-OS 2 024 613 (n.sub.e : 1.482, v: 84).
The glasses, however, due to the presence of B.sub.2 O.sub.3, have lower Abbe values and higher refractive indices.